DIVERSITY IN NITROGEN ACQUISITION MECHANISMS AMONG ISOLATES OF PROCHLOROCOCCUS

Prochlorococcus and Synechococcus have different horizontal and vertical distributions in the sea. Moreover, within Prochlorococcus low-light ecotypes are often found in the deep euphotic zone near the nitracline, whereas high-light ecotypes tend to be found more often in nitrate-depleted surface waters. We compared the capabilities of different Prochlorococcus and Synechococcus isolates to grow on a variety of nitrogen sources. We found that all the isolates grew well on ammonium but that none of the Prochlorococcus ecotypes were able to grow with nitrate. Surprisingly, four low-light isolates grew on nitrite, but all others did not. Whole genome analysis of MED4 and MIT9313 revealed that the genes required for nitrate uptake and reduction were absent from both. The genome of MIT9313, one of the low-light ecotypes that grew on nitrite, carries an ORF with >80% sequence similarity to the Synechococcus WH8103 nitrite reductase gene. Thus, the utilization of different nitrogen sources in the marine environment is partitioned among closely related ecotypes, each with adaptations optimized for the environment where these sources are available.